1. Field
The present disclosure relates generally to transistors, diodes and fabrication processes for transistors and diodes. More particularly, it relates to an integrated structure comprising transistors, either high electron mobility or double-heterostructure field effect, and Schottky diodes and a process for fabricating the same.
2. Description of the Related Art
Gallium-Nitride (GaN) microwave monolithic integrated circuit (MMIC) high-electron-mobility-transistor (HEMT) technology has the advantages of high breakdown voltage, high operation temperature, high operation speed and a potentially high degree of integration. Such technology can be used for high-voltage, high-temperature, or high-radiation applications, including automotive, aviation and aerospace systems. Applications requiring high breakdown voltages, such as DC-to-DC switching power supplies, can be implemented with the GaN integrated circuit process.
However, the gate/source Schottky diode (one of the key components used in digital, analog or mixed-mode circuit) in the existing GaN HEMT process suffers from excessive parasitic capacitance and resistance due to its lateral junction structure, and non-ideal semiconductor layer structure. This parasitic capacitance and resistance leads to lower operation speed, lower gain, higher power consumption, and larger physical size. The non-ideal larger structure also leads to large and variable non-ideality factors in the Schottky diode I-V characteristics. Schottky diodes are semiconductor diodes with a low forward voltage drop and a very fast switching action. They are well known to the person skilled in the art and will not be described here in detail.
FIG. 1 shows a mesa structure obtained in accordance with a prior art GaN MMIC HEMT process. The structure comprises a plurality of HEMTs 20 separated by isolation elements 30. The HEMTs 20 share a substrate 1, for example a semi-insulating SiC substrate.
Each HEMT 20 comprises an undoped (i.e. unintentionally doped) semiconductor layer 2, e.g. a GaN buffer layer having a bandgap that is less than the band gap of semiconductor layer 5 later described. Within the undoped layer 2 there is generated a two-dimensional electron gas 3 in the structure, which forms the HEMT transistor channel. An undoped ‘spacer’ semiconductor layer 4 is disposed on top of the undoped layer 2 to further enhance the carrier mobility in the two-dimensional electron gas 3. The spacer layer 4 can be made of Al0.25Ga0.75N. The Al mole fraction of any AlXGa1-XN layer could typically be in the range of 0.15 to 0.40.
The embodiment shown in FIG. 1 may also comprise a donor semiconductor layer 5 having a band gap and be optionally doped with a charge carrier (e.g., an N-type dopant in the form of silicon). Usually the donor layer 5 is made of Al0.25Ga0.75N.
An undoped spacer layer is deposited on top of the donor layer 5. It can be made of Al0.25Ga0.75N. If the donor layer 5 is undoped, then two spacer layers 4, 6 and the donor layer 5 can be formed as one contiguous spacer layer 4, 5, 6.
A source ohmic contact 7 and drain ohmic contact 8 are formed on top of the structure. The first cap layers 9, 10 are placed as to avoid direct contact between the ohmic contacts 7, 8 and the spacer layer 6. The first cap layers 9, 10 can be made of N+ doped GaN. (N+doping is typically considered to be in the range of 1E17 to 1E19 cm−3; doping in the range of 0 to 1E18 cm−3 is typically considered N−.) The structure also comprises second cap layers 11, 12, which can be made of N+ doped Al0.25Ga0.75N.
An e-beam resist 13 is formed on the structure, and a metallic gate 14 is evaporated into a gate pattern formed by the resist 13. The voltage on gate 14 controls the two-dimensional electron gas 3.
A Schottky diode can be formed in the structure shown in FIG. 1. In particular, the drain and source terminals 7, 8 are shorted together as the cathode of the diode. The gate terminal 14 forms the anode of the diode. When operating as a Schottky diode, formation of the diode occurs by way of the junction between gate 14 and channel layer 6 and the currents flow laterally in layer 6 from the diode junction to the cathode.
While the undoped layer 6 provides for high carrier mobility underneath the gate 14, the low carrier concentration leads to a high series resistance between the gate 14 and the cathode contact 7, 8, resulting in increased loss in switching and level shifting applications. With this conventional gate Schottky structure, reductions in this resistance can only be achieved with wider devices, a one-for-one trade-off between the resistance and parasitic capacitance that ultimately limits the switching speed of the diode.
An added problem resulting from the high resistivity of the channel layer 6 is that of a “current crowding” when the gate Schottky is forward-biased. A lateral I·R voltage drop along the channel results in the middle of the gate Schottky diode being less forward-biased than the edges, so that there is more current conduction along the edges of the diode than in the middle. Thus, beyond a certain point, increasing the channel length to increase the diode area will not appreciably affect the current-handling capability of the diode. Once again, the only alternative is to increase the device width, leading to a trade-off between parasitic capacitance and current handling capability—a second key limitation of the conventional gate Schottky diode.
Therefore, the Schottky diode (one of the key components used in digital, analog, or mixed-mode circuits) in the existing GaN MMIC HEMT process suffers from excessive parasitic capacitance and resistance due to its lateral junction structure. This parasitic capacitance and resistance lead to lower operation speed, lower gain, higher power consumption, and larger physical size.
Another prior art structure is shown in Gerard T. Dang at al, “High Voltage GaN Schottky Rectifiers”, IEEE Trans. On Electron Devices, Vol. 47, No. 4, April 2000. The Schottky diode process described by Dang at al. does not integrate GaN HEMT transistors on the same wafer.
The GaN MMIC HEMT process was designed for microwave and millimeter-wave amplifier applications. Integration levels for microwave amplifiers are not very high (e.g., 2 to 4 HEMT transistors). As the integration complexity of GaN HEMT integrated circuits becomes higher, the performance of Schottky diodes (as voltage shifters and rectifiers) becomes more important. There is a need for high performance Schottky diodes to make GaN HEMTs more competitive.